heterodyne: Ensure that spectral cubes have a VARIANCE component.

CADC demands that certain data products have a VARIANCE component.
Some early (2006--2007) ACSIS data have crazy negative Tsys values
and MAKECUBE is unable to generate variance from the spread of values.
This change creates a VARIANCE component of bad values so that the
group products meet CADC ingestion requirements.

This deficiency was only revealed after other fixes enabled the
corresponding recipe instances to complete.
heterodyne: Document the flat-field velocity limits in recipes.